This invention relates to improved methods, apparatus, and computer program products for applications such as processing workpieces, more particularly, processing workpieces for electronic device fabrication.
Plasma etching is a vital processing step in the manufacture of integrated circuits. In plasma etching, typically species from a glow discharge plasma comprising reactive ions and neutral species are accelerated towards a workpiece such as a silicon wafer. Parts of the wafer are masked by a protective layer of photoresist. The plasma etches the unprotected areas of the silicon wafer.
Generally, plasma-etching processes can generate considerable heat by two mechanisms—exothermic reactions at the wafer surface, and physical bombardment of the wafer by the plasma. In addition, etching processes can be very sensitive to temperature. For these two reasons, the silicon wafer is “chucked” by placing it in intimate thermal contact with a cooling chuck that is maintained at a steady temperature. By chucking the wafer, heat generated at the wafer surface can be safely removed to the chuck. In addition, chucking contributes to maintaining the wafer at a uniform temperature set point. Thermal contact between the wafer and the chuck is often assisted using helium gas in the gap between the wafer and the chuck.
For effective equipment design, process optimization, or fault detection and isolation, it is desirable to deduce the pattern of heat flows from the plasma to the wafer, and from the wafer to the chuck. It is conceivable that thermal flux sensors such as Gardon gauges could be arranged in an array on the wafer surface to determine the thermal flux from the plasma. These sensors directly sense the thermal flux, typically by measuring the temperature drop across a distance. A second set of thermal flux sensors can be placed near the bottom surface of the wafer to determine the thermal flux to the chuck. Alternately, the thermal flux to the chuck could be deduced from sensors mounted on the chuck, but this would interfere with the chucking operation. These approaches using thermal flux sensors have not been tried before. For plasma applications, these approaches would require using wireless arrays of thermal flux sensors. Commercially available thermal flux sensors would not be suitable, as they need external power and a means to access the measured fluxes. Recently developed MEMS based structures could be used but they would be expensive.
Clearly, there are numerous applications requiring reliable and efficient methods and apparatus for deriving thermal flux information. For processing workpieces that involves several heat sources and/or heat sinks, there is a need for methods and apparatus capable of resolving the net heat flux to the workpiece into components of heat flux from the various heat sources or sinks. Examples of important applications are the plasma processing of workpieces such as semiconductor wafers, flatpanel displays, lithography masks, and other electronic devices.